The disclosures of Japanese Utility Model Application Laid-Open (JP-U) No. S63-112947 and Japanese Patent Application Laid-Open (JP-A) No, S63-101180 are, for example, already known in this technical field. In JP-U No. S63-112947, the opening face of a water discharge port formed at the bottom end of a drain pipe is sloped, and a valve that opens and closes such a water discharge port is configured to swing about a fixed upper portion. In JP-A No. S63-101180, a water absorbing rubber swells due to the process of water that has accumulated at the inside of a floor panel flowing downward from a plate into a chamber section. A valve body provided at the lower end of the chamber section is thereby press downward, so as to open a water discharge hole. An example of a hole plug not provided with a valve or valve body to open and close an opening hole is given in Japanese Patent No. 4409548.